Method and Apparatus for Disabling High Speed Bus Operation Under High Common Mode Voltage Conditions

ABSTRACT

An apparatus including a handshake window enabler having a pair of differential inputs and a window enablement output, a common mode detector coupled to a power input and a ground input and having a handshake inhibit output, and a handshake disabler coupled to the handshake window enabler, the common mode detector, and the pair of differential inputs. If a common mode voltage that out of range (“too high”) is detected, high speed handshake protocols are such that the bus operates a lower data rate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of co-pending U.S. Provisional Patent Application No. 62/402,843, filed Sep. 30, 2016, which is incorporated herein by reference.

BACKGROUND

The Universal Serial Bus (USB) is an industry standard protocol developed in the mid-1990s that defines cables, connectors and communication protocols used in a bus for connection, communication and power supply between a computer (host) and an electronic device (client). Unlike other data buses (e.g., Ethernet, HDMI), USB connections are directed, with both upstream and downstream ports emanating from a single host. It is currently developed by the USB Implementers Forum (USB IF).

The original USB 1.0 specification, which was introduced in January 1996, defined data rates of 1.5 Mbit/s as “Low Speed” and 12 Mbit/s as “Full Speed.” The USB 2.0 specification was released in April 2000 and was ratified by USB-IF at the end of 2001 and defined an additional data rate of 480 Mbit/s as “High Speed.” USB 2.0 uses four shielded wires: two for power (VBUS and GND) and two for differential data signals (D+ and D−), providing half-duplex data transfers. The data rate for a USB 2.0 bus defaults to Full Speed after each bus reset, but can be increased to High Speed as determined by a “handshake” protocol using out-of-band signals to verify that both host and client are capable of High Speed operation.

While the D+ and D− signals are transmitted on a differential pair with USB 2.0, they still may be affected by large common-mode differentials between the host and client when, for example, the grounds of the host and/or client aren't at earth ground. This can happen in industrial settings, on airplanes, with heating, venting and air conditioning (HVAC) systems, to name a few.

Common-mode interference becomes more problematical at higher data rates, e.g. during High Speed operation of a USB 2.0 bus. There are therefore situations where it is desirable to operate a USB 2.0 bus at the slower Full Speed rate even though the devices are High Speed compatible. However, the automatic handshake protocol used after a bus reset makes Full Speed operation problematical when both the host and client are High Speed compatible.

These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.

SUMMARY

A method and apparatus are provided for disabling High Speed bus operation in a USB 2.0 bus.

An example method for disabling High Speed bus operation includes monitoring a Universal Serial Bus (USB) for a bus reset signal, and disabling a High Speed handshake protocol on the USB during a handshake window period after a detection of a reset if a High Speed disable signal is present. The High Speed disable signal can be the result of the detection of a high common mode voltage on the bus or can be for other reasons where lower data rates are desired.

A first example apparatus includes handshake window enabler having a pair of differential inputs and a window enablement output, a common mode detector coupled to a power input and a ground input and having a handshake inhibit output, and a handshake disabler coupled to the handshake window enabler, the common mode detector, and the pair of differential inputs. If a common mode voltage that out of range (“too high”) is detected, High Speed handshake protocols will be disabled.

A second example apparatus includes: a handshake window enabler having a pair of differential inputs and a window enablement output, wherein the handshake window enabler includes a USB reset detector and a one shot multivibrator coupled to the USB reset detector; a common mode detector coupled to a power input and a ground input and having a handshake inhibit output, wherein the common mode detector includes a ground line resistor provided between the ground input and a ground output, a ground comparator coupled to the ground input, a power comparator coupled to the power input, and a latch having a set input coupled to the ground comparator and a reset input coupled to the power comparator; a handshake disabler coupled to the pair of differential inputs; and an AND gate having inputs coupled to the window enablement output and the handshake inhibit output and an output coupled to a control input of the handshake disabler.

These and other embodiments, features and advantages will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

Several example embodiments will now be described with reference to the drawings, wherein like components are provided with like reference numerals. The example embodiments are intended to illustrate, but not to limit, the invention. The drawings include the following figures:

FIG. 1 is a block diagram of a first example embodiment of an apparatus for disabling High Speed bus operation under high common mode voltage conditions;

FIG. 2 is a block diagram of a second example embodiment of an apparatus for disabling High Speed bus operation under high common mode voltage conditions;

FIG. 3 is a flow diagram illustrating an example common mode detection process;

FIG. 4 is a flow diagram illustrating an example High Speed inhibit process;

FIG. 5 is a timing diagram illustrating a High Speed handshake process on a USB 2.0 bus; and

FIG. 6 is a timing diagram illustrating the High Speed handshake process being inhibited on a USB 2.0 bus.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In FIG. 1, an apparatus 10 for disabling High Speed bus operation, set forth by way of example and not limitation, includes a handshake window enabler 12, a common mode detector 14, and a handshake disabler 16. The handshake window enabler 12 has a pair of differential inputs D+_IN and D−_IN and a window enablement output WE. The common mode detector 14 is coupled between a power input VBUS_IN and a ground input GND_IN and has a handshake inhibit output HS_Inhibit. The handshake disabler 16 is coupled to the WE output of handshake window enabler 12 and to the HS_Inhibit output of common mode detector 14 by an AND gate 18 and to the differential pair D+_IN and D−_IN.

The handshake window enabler 12, in this non-limiting example, includes a USB Reset Detector 20 and a One Shot Multivibrator (One Shot) 22. THE USB Reset Detector 20 has inputs coupled to the D+_IN and D−_IN lines and an output coupled to an input of the One Shot 22. The output of One Shot 22 develops the window enablement signal WE. In this non-limiting example, the USB Reset Detector detects a bus reset when both D+_IN and D−_IN are LO for approximately 2.5 microseconds and triggers the One Shot 22 output WE to go HI for approximately 9 milliseconds. In this example, the 9 milliseconds comprises a “handshake window” during which High Speed handshakes can occur.

The common mode detector 14 includes, in this non-limiting example, a ground line resistor 24 between GND_IN and GND_OUT, a pair of comparators 26 and 28, an OR gate 30, a latch 32 and a comparator 34. In this example, the comparators 26 and 28 are referenced to +0.25 V and −0.25V such that the output of OR gate 30 is HI if GND_IN is out of the range of −0.25V to +0.25V (a range of acceptable common mode voltages). If the common mode voltages are out of range, OR Gate 30 will set latch 32 such that the HS_Inhibit signal is HI. The latch 30 is reset by comparator 34 when VBUS_IN crosses, for example, a 3V reference voltage. The term “high common mode voltage”, as used herein, means a high absolute value for the common mode voltage, either negative or positive.

The handshake disabler 16 includes a normally closed, single-throw, double pole electronic switch 36, a normally open, single-throw, quadruple pole electronic switch 38, four grounding resistors 40, and an inverter 42. In this non-limiting example, electronic switch 36 includes a first differential line electronic switch coupling a first differential input D+_IN to a first differential output D+_(—) OUT and a second differential line electronic switch coupling a second differential input D−_IN to a second differential output D−_OUT. Also in this non-limiting example, electronic switch 38 includes a first electronic switch, a second electronic switch, a third electronic switch and a fourth electronic switch, each of which forms a series connection with one of the four grounding resistors 40. For example, the handshake disabler 16 can include a series connection of a first electronic switch and a first resistor coupling a first differential input of the pair of differential inputs to ground, a series connection of a second electronic switch and a second resistor coupling a second differential input of the pair of differential inputs to ground, a series connection of a third electronic switch and a third resistor coupling the first differential output to ground, and a series connection of a fourth electronic switch and a fourth resistor coupling the second differential input of the pair of differential inputs to ground.

In operation, when the output HD of AND gate 18 is HI (e.g. WE is HI and HS_Inhibit is HI resulting in a handshake disable signal), electronic switch 36 opens and electronic switch 38 closes to isolate and ground each of D+_IN, D+_OUT, D−_IN and D−_OUT. This effectively stops any handshake communication on the D+ and D− lines for the duration of the handshake window. When the output HD of AND gate 18 is LO, electronic switch 36 closes and electronic switch 38 opens, allowing normal communication on the D+ and D− lines.

In FIG. 2, an alternative apparatus 10′ for disabling High Speed bus operation is illustrated, where like reference numerals are used for like devices in FIGS. 1 and 2. In this second example embodiment, apparatus 10′ for the first example embodiment only in the configuration of handshake disabler 16′. In this non-limiting embodiment, when the output HD is HI (e.g. a handshake disable signal), a double-throw, single pole, normally open electronic switch 44 is closed, shorting the D+ and D− lines to ground and preventing any handshake communication for the duration of the handshake window. In this alternate example embodiment, the electronic switch can include a pair of electronic switches coupling the pair of differential inputs to ground, wherein the pair of electronic switches are controlled via the control input of the handshake disabler.

FIG. 3 is a flow diagram illustrating an example common mode detection process 46 which, in this example embodiment, is implemented by the hardware of the common mode detector 14. Common mode detection idles at 48 with HS_Inhibit set at “0” or “LO” until VBUS exceeds 3V. Next, in an operation 50, the common mode voltage is checked to see if it is in range, e.g. from −0.25V to +0.25V. If the common mode voltage is outside of the range, and operation 52 set HS_Inhibit to “1” or “HI”, and if VBUS drops below 3V process control returns to operation 48.

FIG. 4 is a flow diagram illustrating an example High Speed inhibit process 54 which, in this example embodiment, is implemented by the hardware of FIG. 1 or 2. The process idles in High Speed Inhibit mode in operation 56 until D− is LO and D+ is falling, at which time a reset is detected at operation 58. When D− is LO and D+ is LO for a period of time (e.g. 2.5 μs) an operation checks for high common mode. If HS_Inhibit is LO, control returns to operation 56. If HS_Inhibit is HI, an operation 62 inhibits High Speed by preventing the High Speed handshake protocol on the D+ and D− busses. After the handshake window (a/k/a 9 millisecond timer) expires, control returns to operation 56.

FIGS. 5 and 6 are timing diagrams illustrating the operation of the apparatus and methods described above. In FIG. 5, the HS_Inhibit is LO and, as can be seen at the point “HS Handshake Start”, the D+ and D− lines are allowed to perform their handshake processes to configure the bus for High Speed operation. In FIG. 6, shortly after USB Reset Start, the HS_Inhibit line is HI (due to the detection of a high common mode voltage, e.g. a common mode voltage that is out of range), and the D+ and D− lines are grounded, preventing any handshaking processes to occur.

While various embodiments have been discussed wherein High Speed operation is disabled when high common mode voltages are present on the bus, there are other reasons why disabling High Speed operation may be desirable. For example, if the host and client don't need high data rate communication, it may be desirable to disable High Speed operation in order to reduce power consumption, reduce radiation, increase bus length, etc. The HS_(—) Inhibit signal can therefore be generated in a number of fashions, including simply tying the HS_Inhibit line to the power bus.

Although various embodiments have been described using specific terms and devices, such description is for illustrative purposes only. The words used are words of description rather than of limitation. It is to be understood that changes and variations may be made by those of ordinary skill in the art without departing from the spirit or the scope of various inventions supported by the written disclosure and the drawings. In addition, it should be understood that aspects of various other embodiments may be interchanged either in whole or in part. It is therefore intended that the claims be interpreted in accordance with the true spirit and scope of the invention without limitation or estoppel. 

What is claimed is:
 1. A method for disabling high speed bus operation comprising: monitoring a Universal Serial Bus (USB) for a bus reset signal; and disabling a high speed handshake protocol on the USB during a handshake window period after a detection of a reset if a high speed disable signal is present.
 2. A method for disabling high speed bus operation as recited in claim 1 further comprising detecting a common mode voltage on a Universal Serial Bus (USB) and developing the high speed disable signal if the common mode voltage is out of range.
 3. An apparatus for disabling high speed bus operation under high common mode voltage conditions comprising: a handshake window enabler having a pair of differential inputs and a window enablement output; a common mode detector coupled to a power input and a ground input and having a handshake inhibit output; and a handshake disabler coupled to the handshake window enabler, the common mode detector, and the pair of differential inputs.
 4. An apparatus for disabling high speed bus operation under high common mode voltage conditions as recited in claim 3 wherein the handshake window enabler includes a USB reset detector and a one shot multivibrator coupled to the USB reset detector.
 5. An apparatus for disabling high speed bus operation under high common mode voltage conditions as recited in claim 3 wherein the common mode detector includes a resistor provided between the ground input and a ground output, a ground comparator coupled to the ground input, a power comparator coupled to the power input, and a latch having a set input coupled to the ground comparator and a reset input coupled to the power comparator.
 6. An apparatus for disabling high speed bus operation under high common mode voltage conditions as recited in claim 3 further comprising an AND gate having inputs coupled to the window enablement output and the handshake inhibit output and an output coupled to a control input of the handshake disabler.
 7. An apparatus for disabling high speed bus operation under high common mode voltage conditions as recited in claim 3 wherein the handshake disabler include a pair of electronic switches coupling the pair of differential inputs to ground, wherein the pair of electronic switches are controlled via the control input of the handshake disabler.
 8. An apparatus for disabling high speed bus operation under high common mode voltage conditions comprising: a handshake window enabler having a pair of differential inputs and a window enablement output, wherein the handshake window enabler includes a USB reset detector and a one shot multivibrator coupled to the USB reset detector; a common mode detector coupled to a power input and a ground input and having a handshake inhibit output, wherein the common mode detector includes a ground line resistor provided between the ground input and a ground output, a ground comparator coupled to the ground input, a power comparator coupled to the power input, and a latch having a set input coupled to the ground comparator and a reset input coupled to the power comparator; a handshake disabler coupled to the pair of differential inputs; and an AND gate having inputs coupled to the window enablement output and the handshake inhibit output and an output coupled to a control input of the handshake disabler.
 9. An apparatus for disabling high speed bus operation under high common mode voltage conditions as recited in claim 8 wherein the handshake disabler includes a pair of electronic switches coupling the pair of differential inputs to ground, wherein the pair of electronic switches are controlled via the control input of the handshake disabler.
 10. An apparatus for disabling high speed bus operation under high common mode voltage conditions as recited in claim 9 wherein the pair of electronic switches comprise a double-throw, single pole, normally open electronic switch.
 11. An apparatus for disabling high speed bus operation under high common mode voltage conditions as recited in claim 8 wherein the handshake disabler includes a series connection of a first electronic switch and a first resistor coupling a first differential input of the pair of differential inputs to ground, and a series connection of a second electronic switch and a second resistor coupling a second differential input of the pair of differential inputs to ground.
 12. An apparatus for disabling high speed bus operation under high common mode voltage conditions as recited in claim 11 wherein the handshake disabler further includes a first differential line electronic switch coupling the first differential input to a first differential output and a second differential line electronic switch coupling the second differential input to a second differential output.
 13. An apparatus for disabling high speed bus operation under high common mode voltage conditions as recited in claim 12 wherein the first differential line electronic switch and the second differential line electronic switch comprise a normally closed, single-throw, double pole differential electronic switch.
 14. An apparatus for disabling high speed bus operation under high common mode voltage conditions as recited in claim 13 further comprising an inverter coupling the output of the AND gate to a control input of the normally closed, single-throw, double pole differential electronic switch.
 15. An apparatus for disabling high speed bus operation under high common mode voltage conditions as recited in claim 12 wherein the handshake disabler includes a series connection of a third electronic switch and a third resistor coupling the first differential output to ground, and a series connection of a fourth electronic switch and a fourth resistor coupling the second differential input of the pair of differential inputs to ground.
 16. An apparatus for disabling high speed bus operation under high common mode voltage conditions as recited in claim 15 wherein the first electronic switch, the second electronic switch, the third electronic switch, and the fourth electronic switch comprise a normally open, single-throw, quadruple pole electronic switch.
 17. An apparatus for disabling high speed bus operation under high common mode voltage conditions as recited in claim 16 wherein the output of the AND gate is coupled to a control input of the normally open, single-throw, quadruple pole electronic switch. 